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/*
* Copyright 2014 The gRPC Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package io.grpc;
import javax.annotation.Nullable;
/**
* An instance of a call to a remote method. A call will send zero or more
* request messages to the server and receive zero or more response messages back.
*
* Instances are created
* by a {@link Channel} and used by stubs to invoke their remote behavior.
*
*
More advanced usages may consume this interface directly as opposed to using a stub. Common
* reasons for doing so would be the need to interact with flow-control or when acting as a generic
* proxy for arbitrary operations.
*
*
{@link #start} must be called prior to calling any other methods, with the exception of
* {@link #cancel}. Whereas {@link #cancel} must not be followed by any other methods,
* but can be called more than once, while only the first one has effect.
*
*
No generic method for determining message receipt or providing acknowledgement is provided.
* Applications are expected to utilize normal payload messages for such signals, as a response
* naturally acknowledges its request.
*
*
Methods are guaranteed to be non-blocking. Not thread-safe except for {@link #request}, which
* may be called from any thread.
*
*
There is no interaction between the states on the {@link Listener Listener} and {@link
* ClientCall}, i.e., if {@link Listener#onClose Listener.onClose()} is called, it has no bearing on
* the permitted operations on {@code ClientCall} (but it may impact whether they do anything).
*
*
There is a race between {@link #cancel} and the completion/failure of the RPC in other ways.
* If {@link #cancel} won the race, {@link Listener#onClose Listener.onClose()} is called with
* {@link Status#CANCELLED CANCELLED}. Otherwise, {@link Listener#onClose Listener.onClose()} is
* called with whatever status the RPC was finished. We ensure that at most one is called.
*
*
Usage examples
* Simple Unary (1 request, 1 response) RPC
*
* call = channel.newCall(unaryMethod, callOptions);
* call.start(listener, headers);
* call.sendMessage(message);
* call.halfClose();
* call.request(1);
* // wait for listener.onMessage()
*
*
* Flow-control in Streaming RPC
*
* The following snippet demonstrates a bi-directional streaming case, where the client sends
* requests produced by a fictional makeNextRequest() in a flow-control-compliant
* manner, and notifies gRPC library to receive additional response after one is consumed by
* a fictional processResponse().
*
*
* call = channel.newCall(bidiStreamingMethod, callOptions);
* listener = new ClientCall.Listener<FooResponse>() {
* @Override
* public void onMessage(FooResponse response) {
* processResponse(response);
* // Notify gRPC to receive one additional response.
* call.request(1);
* }
*
* @Override
* public void onReady() {
* while (call.isReady()) {
* FooRequest nextRequest = makeNextRequest();
* if (nextRequest == null) { // No more requests to send
* call.halfClose();
* return;
* }
* call.sendMessage(nextRequest);
* }
* }
* }
* call.start(listener, headers);
* // Notify gRPC to receive one response. Without this line, onMessage() would never be called.
* call.request(1);
*
*
* DO NOT MOCK: Use InProcessServerBuilder and make a test server instead.
*
* @param type of message sent one or more times to the server.
* @param type of message received one or more times from the server.
*/
public abstract class ClientCall {
/**
* Callbacks for receiving metadata, response messages and completion status from the server.
*
* Implementations are free to block for extended periods of time. Implementations are not
* required to be thread-safe, but they must not be thread-hostile. The caller is free to call
* an instance from multiple threads, but only one call simultaneously. A single thread may
* interleave calls to multiple instances, so implementations using ThreadLocals must be careful
* to avoid leaking inappropriate state (e.g., clearing the ThreadLocal before returning).
*
* @param type of message received.
*/
public abstract static class Listener {
/**
* The response headers have been received. Headers always precede messages.
*
* Since {@link Metadata} is not thread-safe, the caller must not access (read or write)
* {@code headers} after this point.
*
* @param headers containing metadata sent by the server at the start of the response.
*/
public void onHeaders(Metadata headers) {}
/**
* A response message has been received. May be called zero or more times depending on whether
* the call response is empty, a single message or a stream of messages.
*
* @param message returned by the server
*/
public void onMessage(T message) {}
/**
* The ClientCall has been closed. Any additional calls to the {@code ClientCall} will not be
* processed by the server. No further receiving will occur and no further notifications will be
* made.
*
*
Since {@link Metadata} is not thread-safe, the caller must not access (read or write)
* {@code trailers} after this point.
*
*
If {@code status} returns false for {@link Status#isOk()}, then the call failed.
* An additional block of trailer metadata may be received at the end of the call from the
* server. An empty {@link Metadata} object is passed if no trailers are received.
*
*
This method should not throw. If this method throws, there is no way to be notified of the
* exception. Implementations should therefore be careful of exceptions which can accidentally
* leak resources.
*
* @param status the result of the remote call.
* @param trailers metadata provided at call completion.
*/
public void onClose(Status status, Metadata trailers) {}
/**
* This indicates that the ClientCall may now be capable of sending additional messages (via
* {@link #sendMessage}) without requiring excessive buffering internally. This event is
* just a suggestion and the application is free to ignore it, however doing so may
* result in excessive buffering within the ClientCall.
*
*
Because there is a processing delay to deliver this notification, it is possible for
* concurrent writes to cause {@code isReady() == false} within this callback. Handle "spurious"
* notifications by checking {@code isReady()}'s current value instead of assuming it is now
* {@code true}. If {@code isReady() == false} the normal expectations apply, so there would be
* another {@code onReady()} callback.
*
*
If the type of a call is either {@link MethodDescriptor.MethodType#UNARY} or
* {@link MethodDescriptor.MethodType#SERVER_STREAMING}, this callback may not be fired. Calls
* that send exactly one message should not await this callback.
*/
public void onReady() {}
}
/**
* Start a call, using {@code responseListener} for processing response messages.
*
*
It must be called prior to any other method on this class, except for {@link #cancel} which
* may be called at any time.
*
*
Since {@link Metadata} is not thread-safe, the caller must not access (read or write) {@code
* headers} after this point.
*
* @param responseListener receives response messages
* @param headers which can contain extra call metadata, e.g. authentication credentials.
* @throws IllegalStateException if a method (including {@code start()}) on this class has been
* called.
*/
public abstract void start(Listener responseListener, Metadata headers);
/**
* Requests up to the given number of messages from the call to be delivered to
* {@link Listener#onMessage(Object)}. No additional messages will be delivered.
*
* Message delivery is guaranteed to be sequential in the order received. In addition, the
* listener methods will not be accessed concurrently. While it is not guaranteed that the same
* thread will always be used, it is guaranteed that only a single thread will access the listener
* at a time.
*
*
If it is desired to bypass inbound flow control, a very large number of messages can be
* specified (e.g. {@link Integer#MAX_VALUE}).
*
*
If called multiple times, the number of messages able to delivered will be the sum of the
* calls.
*
*
This method is safe to call from multiple threads without external synchronization.
*
* @param numMessages the requested number of messages to be delivered to the listener. Must be
* non-negative.
* @throws IllegalStateException if call is not {@code start()}ed
* @throws IllegalArgumentException if numMessages is negative
*/
public abstract void request(int numMessages);
/**
* Prevent any further processing for this {@code ClientCall}. No further messages may be sent or
* will be received. The server is informed of cancellations, but may not stop processing the
* call. Cancellation is permitted even if previously {@link #halfClose}d. Cancelling an already
* {@code cancel()}ed {@code ClientCall} has no effect.
*
*
No other methods on this class can be called after this method has been called.
*
*
It is recommended that at least one of the arguments to be non-{@code null}, to provide
* useful debug information. Both argument being null may log warnings and result in suboptimal
* performance. Also note that the provided information will not be sent to the server.
*
* @param message if not {@code null}, will appear as the description of the CANCELLED status
* @param cause if not {@code null}, will appear as the cause of the CANCELLED status
*/
public abstract void cancel(@Nullable String message, @Nullable Throwable cause);
/**
* Close the call for request message sending. Incoming response messages are unaffected. This
* should be called when no more messages will be sent from the client.
*
* @throws IllegalStateException if call is already {@code halfClose()}d or {@link #cancel}ed
*/
public abstract void halfClose();
/**
* Send a request message to the server. May be called zero or more times depending on how many
* messages the server is willing to accept for the operation.
*
* @param message message to be sent to the server.
* @throws IllegalStateException if call is {@link #halfClose}d or explicitly {@link #cancel}ed
*/
public abstract void sendMessage(ReqT message);
/**
* If {@code true}, indicates that the call is capable of sending additional messages
* without requiring excessive buffering internally. This event is
* just a suggestion and the application is free to ignore it, however doing so may
* result in excessive buffering within the call.
*
*
If {@code false}, {@link Listener#onReady()} will be called after {@code isReady()}
* transitions to {@code true}.
*
*
If the type of the call is either {@link MethodDescriptor.MethodType#UNARY} or
* {@link MethodDescriptor.MethodType#SERVER_STREAMING}, this method may persistently return
* false. Calls that send exactly one message should not check this method.
*
*
This abstract class's implementation always returns {@code true}. Implementations generally
* override the method.
*/
public boolean isReady() {
return true;
}
/**
* Enables per-message compression, if an encoding type has been negotiated. If no message
* encoding has been negotiated, this is a no-op. By default per-message compression is enabled,
* but may not have any effect if compression is not enabled on the call.
*/
public void setMessageCompression(boolean enabled) {
// noop
}
/**
* Returns additional properties of the call. May only be called after {@link Listener#onHeaders}
* or {@link Listener#onClose}. If called prematurely, the implementation may throw {@code
* IllegalStateException} or return arbitrary {@code Attributes}.
*
* @return non-{@code null} attributes
* @throws IllegalStateException (optional) if called before permitted
*/
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/2607")
@Grpc.TransportAttr
public Attributes getAttributes() {
return Attributes.EMPTY;
}
}